Association of triglyceride glucose-related obesity indices with sarcopenia among U.S. adults: a cross-sectional study from the National Health and Nutrition Examination Survey

Data source and study population

The National Health and Nutrition Examination Survey (NHANES) is a comprehensive database utilized for assessing the health and nutritional status of the U.S. population. All protocols were approved by the National Center for Health Statistics Ethics Review Board, ensuring compliance with ethical standards.

This study incorporated data from the NHANES spanning two periods: 1999–2006 and 2011–2018, as these cycles encompassed parameters for skeletal muscle mass. The analytical dataset was extracted directly from the NHANES database and included variables such as age, sex, race, marital status, education level, BMI, height, waist circumference, smoking history, alcohol consumption, history of hypertension, diabetes, skeletal muscle mass, lipid parameters and fasting blood glucose. The exclusion criteria of main analysis included: (1) age < 18 years old; (2) without skeletal muscle mass and BMI; (3) without fasting blood glucose, triglycerides, stand height, weight and waist circumference; (4) without poverty to income ratio (PIR), marital status, education status, race, smoking status, drinking status, hypertension, diabetes, vigorous recreational activities and moderate recreational activities; (5) with cancer or current pregnant (Fig. 1).

Flowchart for inclusion of study participants. NHANES: The National Health and Nutrition Examination Survey; PIR: poverty to income ratio.

Definition of sarcopenia

In the NHANES, sarcopenia is calculated via the appendicular skeletal muscle mass (ASM) index, which is derived from dual-energy X-ray absorptiometry scans, and is adjusted for BMI. The condition is categorized on the basis of the ASM/BMI ratio, with thresholds of less than 0.789 for men and less than 0.512 for women, as defined by the Foundation for the National Institutes of Health sarcopenia project²⁰.

Definitions of TyG- and TyG-related indices

In accordance with the NHANES data collection protocols, triglyceride levels were obtained via the hydrolysed method, with units in milligrams per deciliter (mg/dL). Anthropometric measurements, including height (cm), WC (cm), and weight (kg), were obtained following the standard protocols outlined in the NHANES Anthropometry Procedures Manual. BMI was calculated by dividing weight (kg) by the square of height (m²), and the WHtR was calculated as the waist circumference (cm) divided by the individual’s height (cm)²¹. The TyG index was derived via the following formula: TyG = Ln [triglyceride (mg/dL) × fasting blood glucose (mg/dL)/2]. The TyG-BMI was subsequently determined by multiplying the TyG index by BMI, TyG-WC was subsequently calculated by multiplying TyG by WC, and TyG-WHtR was ascertained by multiplying TyG by WHtR, providing a comprehensive set of metabolic and anthropometric indicators for our analysis.

Covariates

The selection of covariates in our study encompasses demographic factors such as age, sex, ethnicity, education and marital status, as well as lifestyle factors such as smoking status, alcohol consumption, and chronic diseases such as hypertension and diabetes. Anthropometric measurements such as BMI are also included. To deepen our analysis, we stratified the variables into specific categories: BMI was classified into 3 groups: underweight/normal weight (BMI < 25), overweight (25 ≤ BMI < 30), and obese (BMI ≥ 30). Ethnicity will be categorized as Mexican American, other Hispanic, non-Hispanic white, non-Hispanic black, and other races. Education is divided into three categories: less than high school, high school or equivalent, and college or above. Marital status will be categorized as married/living with a partner, previously married/divorced/separated, and never married. Smoking status and alcohol consumption were classified as never smokers/drinkers, ever smokers/drinkers and current smokers/drinkers. Hypertension status, diabetes status, vigorous and moderate recreational activities were classified into two groups: ‘Yes’ and ‘No’.

Statistical analysis approach

In this cardiovascular lipid research, our methodology encompasses a series of statistical analyses to delineate the relationships between TyG-related obesity indices and sarcopenia. We stratified sarcopenia as a binary outcome variable, utilized TyG-related obesity indices both as continuous predictors and were categorically divided into quartiles (Q1-Q4) for dose-response analysis. The general characteristics of the study population were compared between those with and without sarcopenia, and the detailed data are presented in Table 1.

We conducted ordinal logistic regression analyses to assess the associations between TyG-related obesity indices and sarcopenia, employing three models of covariate adjustment. Model 1 was unadjusted, Model 2 was adjusted for age, sex, and race, and Model 3 included a comprehensive adjustment for all covariates. Additionally, to elucidate the potential nonlinear associations, we employed a restricted cubic spline (RCS) function with knots positioned at the 5th, 35th, 65th, and 95th percentiles of the exposure distribution within Model 3. The likelihood ratio test was used to assess the presence of nonlinearity. Finally, we plotted receiver operating characteristic (ROC) curves for TyG, TyG-WC, TyG-BMI and TyG-WHtR for the presence of sarcopenia, with the area under the curve (AUC) providing a measure of predictive accuracy.

We will use appropriate statistical software (R version 4.4.1 with the ‘survey’ package) to account for the complex survey design of the NHANES, including the use of sample weights to ensure national representativeness. Comparisons where P was < 0.05 (two-tailed) were considered statistically significant. All analyses were performed with R (version 4.4.1).